1. Field of the Invention
This invention relates to optical fiber communications transmission networks, and more particularly, relates to a comb splitting system and a method for multiplexing or demultiplexing a plurality of optical bands each containing a plurality of individual channels at different wavelengths.
2. Description of the Related Art
There are several network approaches for implementing optical communication networks between central offices and individual subscribers. These include for example point-to-point networks, power splitting networks, and wavelength division multiplexing networks. In a point-to-point network, one or more optical fibers directly link the central office to each subscriber. In a power splitting network, a transmitter, receiver, and part of an optical fiber in the subscriber loop is shared by many subscribers using power splitters, a wide variety of which are well known in the art. See for an example, U.S. Pat. No. 4,904,042 to Dragone that describes a star coupler, which is a typical power splitter (PS).
A third approach is a wavelength division multiplexing network that employs wavelength division multiplexers (WDM). In this approach, a particular channel (wavelength of carrier) is allocated to each subscriber. Various channels are multiplexed over a single optical fiber and are demultiplexed onto individual fibers associated with each subscriber to create a virtual point-to-point network. A WDM sometimes referred to as a filter or router, is well known in the art and is generally a device that can multiplex and/or demultiplex optical signal wavelengths.
In general, a WDM is usually a passive optical network (PON) element or device with multiple optical paths, each of which exhibits a particular passband, similar to an electrical signal processing filter. The passband permits passage of one or more particular wavelengths along the respective optical path, to the substantial exclusion of others. Thus, the WDM can be used to divide wavelengths of incoming light from a multichannel optical signal or to combine various wavelengths on respective optical paths into one multichannel optical signal on one optical path. For an example of a WDM, see C. Dragone et al., "Integrated Optics N.times.N Multiplexer on Silicon," IEEE Photon. Techno. Lettr., Vol. 3, p. 896 (1989), the disclosure of which is also set forth in U.S. Pat. No. 5,136,671 to Dragone.
As shown in FIG. 1, a Wavelength add and drop (WAD) site consists of an optical path 3 that carries a multichannel optical signal (light) 5. The multichannel optical signal is sent to an optical demultiplexing unit (ODU) 8 which separates the light into various wavelengths (channels) and outputs optical signals into individual predetermined wavelengths (channels) 9, 11, 13. In reverse, the optical multiplexing unit (OMU) 7 combines the multiple incoming signals (channels) into a single multichannel optical signal 5 in a single optical path 3. Various WAD sites 1 may be provided in a cascade network, as shown in FIG. 1, to provide a virtual point-to-point or a ring system.
FIGS. 2 and 3 show a conventional OMU 7 and ODU 8, also referred to as a waveguide grating router (WGR), with an input side 17 and an output side 19. The ODU 8 is composed of an array of single mode waveguides 29 and input 21 and output 23 waveguides connected by two planar slab waveguides 18 and 20. The principles of multiplexing and demultiplexing of the OMU 7 and the ODU 8 are the same, except that light propagation direction is reversed. Light 5 with a plurality of wavelengths 9, 11, 13 is received by the single mode input waveguide 21 and diffracts horizontally in the slab waveguide region 18. Each wavelength propagates through the individual array waveguides 29 and experiences a constant and wavelength-dependent phase shift caused by the path difference. Thus, the phase shift produces a wavelength dependent wavefront tilting, so that light convergence in the output slab waveguide 20 is wavelength dependent. Since every output waveguide 23 is arranged on a circle with radius R/2 (R is shown as 33) and is directed at the center of the output array waveguides, the different wavelength channels in the input light are focused along the focal plane of the output aperture and couple into different output waveguides 23.
Light coupled into the input waveguide 21 and emitted from the WGR must satisfy the grating equation EQU dn.sub.s sin.theta..sub.i +dn.sub.s sin.theta..sub.o +n.sub.c.DELTA.L=m.lambda.
where .theta..sub.i and .theta..sub.o are the diffraction angles at the input 18 and output 20 slabs, respectively; .DELTA.L is the constant optical path length difference between neighboring array waveguides 29; n.sub.s and n.sub.c are effective refractive index of the slab and channel waveguide, respectively; d is the grating pitch 31; and m is the diffraction order and is an integer.
The spatial dispersion is given by ##EQU1##
where the group refractive index n.sub.g is defined according to EQU n.sub.g (.lambda.)=n.sub.c -.lambda..multidot.dn.sub.c /d.lambda.
Since the arrayed grating provides liner dispersion in the wavelength along the focal plane of the output aperture, WDM wavelengths are separated by a distance .DELTA..lambda. dx/d.lambda. along the focal plane at the output angular spacing ##EQU2##
where .DELTA..lambda. is the channel spacing.
The waveguide grating device (WGR) 7 may contain different input and output angular spacings (i.e., .DELTA..theta..sub.i.noteq..DELTA..theta..sub.o), which means that asymmetrical I/O-port design will yield different demultiplexed wavelengths when a signal is input from different input port and is output from different output port. Under this design, the center wavelength of a WGR device can be adjusted by inputting the multiplexed signal at an off-center port. This is referred as the Vernier effect. The proper I/O-port angular ratio and a number of dummy input and output ports 22 can be chosen to compensate the center wavelength offset due to material and processing variations. When the multiplexed signal is input at the i-th port, the demultiplexed wavelength at j-th port is given by EQU .lambda..sub.i.fwdarw.j =.lambda..sub.o +(i+j/R.sub.v).DELTA..lambda.
where R.sub.v is defined as Vernier ratio between the two angular separations at the output port and input port by EQU R.sub.v =.DELTA..theta..sub.o /.DELTA..theta..sub.i
When the output port j=-i, the center wavelength can be shifted by EQU .DELTA..lambda..sub.o =i(1-1/R.sub.v).DELTA..lambda.
.DELTA..lambda. is unchanged, but the center wavelength and the all-wavelength comb are tuned, based on the designed R.sub.v.
A practical WDM network, favors a two stage split of channels that first splits a multichannel optical signal into two channels. Each channel is subsequently demultiplexed into individual channels using a WGR. As shown in U.S. Pat. 5,680,490, by Cohen and Li, a comb splitting system and method for implementing a multistage WDM network are provided for a two-stage split.
The problem with such conventional two-stage splitting systems is that each splitting and combining device (WGR) requires separate environmental controls. This would cause each set of wavelength shifts from each other, which would result in crossed traffic. Thus, while a two stage split is preferable, it is not preferable to have multiple WGRs, one for each secondary split. Additional devices result in increased cost and complexity.
The present claimed invention is directed to further improving such devices by providing an interleaved multi-channel WGR that may be used in a two-stage split.